Coating liquid for forming porous silica

ABSTRACT

The coating liquid for forming porous silica according to the present invention is characterized by preferably containing a partial hydrolysis-condensation product of an alkoxysilane compound, a surfactant and an organic ampholyte, and by having a metal content of not more than 50 ppb. Conventional coating liquids for forming porous silica have such a problem that porous silica films formed therefrom may have poor regularity in micropore alignment when the shelf life of the coating liquids are long. On the contrary, the coating liquid for forming porous silica of the present invention is excellent in self-life stability. Namely, the quality of porous silica formed therefrom is hardly affected by the length of self-life period of the coating liquid. Consequently, the coating liquid is expected to contribute to the stable preparation of porous silica films which cause no shift in capacitance or voltage when exposed to an electric field, have regularly aligned uniform micropores, and are preferably used as an optically functional material or an electronically functional material.

TECHNICAL FIELD

The present invention relates to a coating liquid for forming poroussilica in order to form porous silica which can be used for an opticallyfunctional material, an electronically functional material and the likeor a film thereof.

BACKGROUND ART

In recent years, porous inorganic compounds having uniform micropores(micropore diameter: 2 to 50 nm) has been developed. The porousinorganic compounds have larger micropores, a larger pore volume and alarger surface area than an oxide such as zeolite and the like that hasbeen used from the past so that application of those compounds to acatalyst carrier, a separation adsorbent, a fuel cell, a sensor or thelike has been studied.

As a method for preparing a porous material having these uniformmicropores, a method utilizing control of the structure of an inorganiccompound by the use of an organic compound has been paid attention. Inparticular, it has been known that an oxide having uniform micropores,that is formed by utilizing a cooperative organization(self-organization) due to an interaction between an organic compoundand an inorganic compound, has a larger pore volume and a larger surfacearea than the conventional zeolite and the like.

There has been disclosed a method for preparing a porous material havinguniform micropores utilizing a cooperative self-organization due to aninteraction between an organic compound and an inorganic compound, forexample, in WO 91/11390. Specifically, a method comprising subjecting aprecursor solution comprising a silica gel, a surfactant and the like toa hydrothermal synthesis in a heat-resistant closed vessel to prepareporous silica has been disclosed.

Furthermore, in Bull. Chem. Soc. Jp., Vol. 63, p. 988 (1990), a methodcomprising subjecting kanemite that is a kind of a layered silicate anda surfactant to ion exchange to prepare such a porous material has beendisclosed.

In order to use a porous material having uniform micropores for anoptically functional material, an electronically functional material orthe like, there has been recently reported a method for preparing poroussilica in the form of a film. For example, in Nature, Vol. 379, p. 703(1996), J. Am. Chem. Soc., Vol. 121, p. 7618 (1999) or the like isdescribed a method for forming a film by immersing a substrate in a solsolution comprising a condensation product of alkoxysilanes and asurfactant, and depositing porous silica having uniform micropores onthe substrate surface.

Furthermore, in Chem. Commun., p. 1149 (1996), Supramolecular Science,Vol. 5, 247 (1998), Adv. Mater., Vol. 10, p. 1280 (1998), Nature, Vol.389, p. 364 (1997), Nature, Vol. 398, p. 223 (1999), or the like isdescribed a method for forming a film on a substrate by coating thesubstrate with a coating liquid in which a condensation product ofalkoxysilanes and a surfactant are mixed in an organic solvent, andsubsequently evaporating the organic solvent.

When the porous film having uniform micropores is used as a lowpermittivity insulating film of an integrated circuit, it is requiredthat a metal ion such as an alkali metal including sodium or potassium,or the like is strictly removed. A positively charged ion, when exposedto an electric field, easily moves, because of its drift from apositively biased film to a negatively biased film, causing the shift incapacitance or voltage. Accordingly, in order to obtain a porous filmwhich is preferably used as an insulating film of an opticallyfunctional material and an electronically functional material, it isrequired that a metal, i.e., an impurity is not contained in a coatingliquid used for preparing a porous film. Namely, in order to preparesuch a coating liquid, it needs to remove a metal, i.e., an impurity,from a surfactant, an organic solvent or the like used as a rawmaterial.

However, when a metal ion such as an alkali metal ion or the like isremoved from the coating liquid, zeta potential of a silica oligomer inthe coating liquid is changed, and the mesophase of the silica oligomerand the surfactant becomes unstable. As a result, there has been aproblem such that the regularity in micropores is deteriorated due tothe time from the preparation of a coating liquid until coating.Consequently, a coating liquid such that the regularity in micropores orthe permittivity of the obtained porous silica film becomes uniformregardless of the shelf life, even if a metal ion is removed, has beenin demand.

In order to solve the problem, for example, in JP2002-26003A isdescribed that, even if a metal ion is removed, a porous silica filmhaving uniform micropores is obtained regardless of the shelf life whena coating liquid containing a tetraalkylammonium salt, a tetraorganoammonium salt or an organoamine in an acidic medium is used.

However, there are problems such that the tetralkylammonium salt, thetetraorgano ammonium salt and organoamine are expensive, and organoamineis very toxic.

Furthermore, in Microporous and Mespporous materials, Vols. 35-36, p.545 (2000), organoamine is described, which penetrates into the insideof the surfactant, affecting the regularity in micropores or a poresize. For this reason, when an amine described in JP2002-26003A is usedas a coating liquid, physical properties such as the mechanical strengthor the like of the obtained porous silica can be possibly deteriorated.

Under the circumstances, a coating liquid for forming porous silicahaving the following properties has been in demand,

porous silica which does not cause the shift in capacitance or voltageis obtained even when it is exposed to an electric field;

the regularity in micropores, the permittivity or the mechanicalstrength of the obtained porous silica is not changed regardless of itsself-life time;

the cost is cheap; and

the safety is high.

DISCLOSURE OF THE INVENTION

The present invention is to solve the problems accompanying thebackground art as described above. That is, an object of the presentinvention is to provide a coating liquid for forming porous silica whichcan be used for an optically functional material, an electronicallyfunctional material or the like, which can form a porous film that doesnot practically contain a metal ion, in which the quality of theobtained porous silica film is uniform regardless of the self-lifeperiod (hereinafter referred to as the self-life stability in somecases), and in which a porous silica film having uniform micropores isobtained.

The coating liquid for forming porous silica according to the presentinvention comprises (A) an alkoxysilane compound, (B) a surfactant and(C) an organic ampholyte, and having a metal content of not more than 50ppb.

(C) The organic ampholyte for the aforementioned coating liquid forforming porous silica is preferably (C1) an amino acid and/or peptide.

The coating liquid for forming porous silica preferably contains (C) theorganic ampholyte in the amount of from 0.1 to 6000 ppm.

(A) The alkoxysilane forming the coating liquid for forming poroussilica of the present invention is preferably a partialhydrolysis-condensation product of (A1) an alkoxysilane compound.

The coating liquid for forming porous silica of the present inventionpreferably has the W_(CR) value defined by the following formula in therange of 0.5 to 3:0,W _(CR) =W _(C)/(60.09×M _(Si))   (Formula 1)

wherein W_(C) represents a mass of (B) the surfactant (unit: gram); andM_(Si) represents the molar amount of (A) the alkoxysilane compound interms of silicon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view illustrating a relationship between the shelf life ofthe coating liquid for forming porous silica and the X-ray diffractionmeasurement results of the porous silica film obtained by using theappropriate coating liquid.

FIG. 2 is a view illustrating a relationship between the shelf life ofthe coating liquid for forming porous silica and the dielectric constantmeasurement results of the porous silica film obtained by using theappropriate coating liquid.

THE BEST MODE FOR CARRYING OUT THE INVENTION

The coating liquid for forming porous silica having excellent self-lifestability according to the present invention is described in more detailbelow. Furthermore, in the present invention, the meaning of a liquidincludes a solution, a suspension, an emulsion and the like in somecases.

[Coating Liquid for Forming Porous Silica]

The coating liquid for forming porous silica according to the presentinvention (hereinafter referred to as a coating liquid) comprises (A) analkoxysilane compound, (B) a surfactant and (C) an organic ampholyte.Alkoxysilane compounds and surfactants that have been used for theconventional coating liquids for forming porous silica can be used for(A) the aforementioned alkoxysilane compound and (B) the surfactantwithout restrictions. However, (C) the organic ampholyte or an organicamide compound to be described below is not included in the conventionalcoating liquid for porous silica.

As a preferred method for obtaining this coating liquid, a methodcomprising mixing (A) an alkoxysilane compound, (B) a surfactant, anacid catalyst, water, an organic solvent and (C) an organic ampholyte,and subjecting (A) the alkoxysilane compound to a partialhydrolysis-dehydrative condensation can be cited. As far as it is notcontrary to the object of the present invention, other components inaddition to the above components may be contained in the aforementionedcoating liquid. For example, an amide compound or the like is preferablyused in forming a porous silica film as it can enhance the smoothness ofa film.

In order to obtain porous silica which does not cause the shift incapacitance or voltage, the metal content of the coating liquid of thepresent invention needs to be not more than 50 ppb.

When the coating liquid of the present invention is used for forming aporous silica film, the coating liquid preferably maintains its fluidityto such an extent that a film can be formed when coating the surface ofa substrate such as a wafer or the like by a spin coating method, a dipcoating method or the like. Furthermore, it is also a preferredembodiment that a liquid is at a state that a locally condensed gel typeparticle affecting the smoothness of a film is not formed.

The meaning of a metal content of a coating liquid in the presentinvention include the content of all metal ions which are generallydefined as a metal such as an alkali metal ion, an alkaline-earth metalion, a transition metal ion or the like.

Usually, when the metal content of the coating liquid becomes not morethan 50 ppb, and the self-life period of the coating liquid is long,there are some defects such that a film formed therefrom may haveremarkably poor regularity in micropores and the like in many cases.

The present inventors have repeatedly conducted an extensive study inorder to overcome the defects. As a result, they have found that, when(C) the organic ampholyte is present in the coating liquid, even if ametal content is not more than 50 ppb, the foregoing defects have beenconsiderably corrected. Namely, even-when the coating liquid is kept fora long period of time, a film formed therefrom has high regularity inmicropores.

As described above, there has been reported that a coating liquidcontaining a tetraalkylammonium salt, a tetraorgano ammonium salt,organoamine or the like can also be stored for a long period of time.However, as compared to these compounds, (C) the aforementioned organicampholyte can be available with a cheaper price and it is highly safe aswell. Furthermore, the coating liquid of the present invention has muchhigher long-term self-life property.

The reason why the liquid stability of the coating liquid of the presentinvention is kept for a long period of time is assumed as follows. Therehas been reported that the compound such as the aforementionedorganoamine can suppress a change in zeta potential of a silicaprecursor in the coating liquid, but it penetrates into the inside of amicelle of a surfactant, resulting in influencing on the regularity inmesopores or its size. On the contrary, since (C) the organic ampholyteused for the coating liquid of the present invention has high polarity,the compound is expected not to penetrate into the aforementionedmicelle or its difficulties in penetrating into the micelle areexpected. For this reason, (C) the organic ampholyte is considered toshow only an effect of suppressing a change in zeta potential withoutinterrupting functions that controls the regularity in mesopores or itssize of (B) the surfactant.

Each component used for the preparation of the coating liquid forforming porous silica in the present invention is explained in moredetail below.

((A) Alkoxysilane Compound)

Alkoxysilane that has been used for coating liquids from the past can beused for (A) an alkoxysilane compound constituting the coating liquid ofthe present invention without any particular restrictions. Thealkoxysilane compound preferably comprises alkoxysilane having Si—O—Sibonds. A partial hydrolysis-condensation product of (A1) alkoxysilane ismore preferable.

As (A) the alkoxysilane compound to be used for the present invention,an alkoxysilane compound represented by the following general formula(I) and/or the following general formula (II) can be particularlypreferably cited,(C_(Y)H_(2Y+1)O)_(4-n)Si((CH₂)_(a)(CF₂)_(b)(O(CF₂)_(c))_(d)X)_(n)   (I)

wherein, in the formula, Y is an integer of 1 to 4; n is an integer of 0to 3; a is an integer of 0 to 3; b is an integer of 0 to 10; c is aninteger of 1 to 3; d is an integer of 0 to 3; and X represents any oneof F, OCF₃, OCF(CF₃)₂, OC(CF₃)₃ and C₆H_(e)F_((5-e)) (in the formula, eis an integer of 0 to 4), and(C_(z)H_(2x+1)O)₃SiRSi(OC_(z)H_(2z+1))₃   (II)

wherein, in the formula, Z is an integer of 1 to 4; and R represents analkyl group or a phenyl group.

As the alkoxysilane compound, an alkoxysilane compound as described inJP2003-89513A and the like can be preferably cited.

In the present invention, (A) the alkoxysilane compound can also be usedin combination of 2 or more kinds. Use of tetraethoxysilane isparticularly preferable. By using tetraethoxysilane, it is possible toeasily control the hydrolysis reaction at a room temperature.

An alkoxysilane compound which does not contain a metal is preferablyused for (A) the alkoxysilane compound according to the presentinvention. As a method for obtaining (A) the alkoxysilane compound asdescribed above, a method comprising removing a metal from thealkoxysilane compound by carrying out distillation purification or ionexchange can be properly cited. Furthermore, a product that has beencommercialized as a grade for an electronic material can also be used asit is.

((B) Surfactant)

The (B) surfactant to be used for the preparation of the coating liquidof the present invention is not particularly restricted and allsurfactants that have been used for coating liquids from the past can bepreferably used. A surfactant having a polyalkyleneoxide structure canbe more preferably cited. Concrete examples of a surfactant having apolyethyleneoxide structure, a polypropyleneoxide structure, apolytetramethyleneoxide structure, a polybutyleneoxide structure and thelike include a block copolymer of the aforementioned polyalkyleneoxide,alkylether of the above polyalkyleneoxide, or the like.

More concrete examples of (B) the surfactant include a surfactant asdescribed in JP2003-89513A and the like.

In the present invention, the surfactants can also be used incombination of 2 or more kinds. Furthermore, the surfactant in any stateof a solid, an undefined phase, a liquid, a solution or the like may begood.

As (B) the surfactant according to the present invention, a surfactantwhich does not contain a metal is preferably used. As a method forobtaining the surfactant as described above, a method comprisingremoving a metal by carrying out ion exchange using a commercial cationexchange resin can be properly cited. Furthermore, a product that hasbeen commercialized as a grade for an electronic material can also beused as it is.

The coating liquid for forming porous silica of the present inventionhas the content of (A) the alkoxysilane compound and (B) the surfactantpreferably in the range of 0.5 to 3.0 of the W_(CR) value represented bythe following formula 1, and more preferably in the range of 0.5 to 2.0,W _(CR) =W _(C)/(60.09×M _(Si))   (Formula 1)

wherein, W_(C) is a mass of (B) the surfactant (unit: gram); and M_(Si)is the molar amount of (A) the alkoxysilane compound in terms ofsilicon.

When the porous silica to be described later is prepared by using thecoating liquid for forming porous silica of the present invention, mostof silicon in (A) the alkoxysilane compound is considered to be siliconin the porous silica. The general composition formula of silica is SiO₂(molecular weight: 60.09), while the aforementioned W_(CR) value can beconsidered as an index that defines the weight part of (B) thesurfactant necessary for preparing 1 weight part of the porous silica.

The content of (B) the surfactant contained in the coating liquid forforming porous silica of the present invention is rather preferablydefined as a molar ratio to the (A) the alkoxysilane compound in termsof a silicon atom in some cases. In this case, (B) the surfactant isused to have the molar ratio of preferably from 0.003 to 0.20, morepreferably from 0.003 to 0.10, more preferably from 0.003 to 0.05, andfurther preferably 0.005 to 0.03, and particularly preferably from 0.007to 0.02, based on the molar ratio to the (A) the alkoxysilane compound.The aforementioned range of the molar ratio is proper in many cases whena surfactant having a relatively large molecular weight such as a blockcopolymer of the above-mentioned polyalkyleneoxide or the like is used.

The porous silica coating liquid in which (B) the surfactant accordingto the present invention satisfies the above range highly enhances theregularity in micropore structure or the void fraction (porosity) inmany cases, since the ratio of silica incapable of contributing to acooperative organization due to an interaction between the alkoxysilanecompound and the surfactant is reduced. Furthermore, it is alsoadvantageous from the viewpoint of forming a hexagonal periodic crystalstructure having uniform micropores, while it is advantageous that thestructure hardly collapses in many cases even in the calcining processwhen forming the porous silica to be described later.

(Acid Catalyst, Water, and Organic Solvent)

In order to obtain a partial hydrolysis-condensation product of theabove alkoxysilane, an acid catalyst, water, and an organic solvent arepreferably used. As the aforementioned acid catalyst, water, and theorganic solvent, all known compounds that have been used for thepreparation of coating liquids from the past can be used without anyrestrictions.

((C) Organic Ampholyte)

(C) The organic ampholyte constituting the coating liquid of the presentinvention is assumed to have an ability to stabilize zeta potential ofan alkoxysilane compound which becomes unstable by removing a metalelement, thereby resulting in considerably enhancing the self-lifestability of the coating liquid. Furthermore, (C) the organic ampholyteto be used in the present invention is different from (B) thesurfactant.

As preferred examples of (C) the organic ampholyte according to thepresent invention, there can be exemplified, for example, an amino acid,peptide, i.e., a polymer of the amino acid and the like can be cited.The amino acid or peptide has at least two or more of acid dissociationconstants in a solvent with ion strength (mol/dm⁻³) of from 0 to 0.2.The acid dissociation constants thereof are preferably included both inthe range of 0 to 4 and in the range of 7 to 13. (C) The organicampholyte according to the present invention can also be used incombination of 2 or more kinds. For example, it can be used incombination of an amino acid and peptide. At this time, the amino acidand peptide may be reacting with each other.

As (C) the organic ampholyte according to the present invention, anamino acid is particularly preferable because it is cheap and highlysafe as well.

As an amino acid which can be used for the present invention, there canbe exemplified, for example, azaserine, asparagine, aspartic acid, aminobutyric acid, alanine, arginine, alloisoleucine, allothreonin,isoleucine, ethionine, ergothioneine, ornithine, canavanine, kynurenine,glycine, glutamine, glutamic acid, creatine, sarcosine, siltathionine,cystine, cysteine, cysteine acid, citrulline, serine, taurin, thyroxine,tyrosine, tryptophan, threonin, norvaline, norleucine, valine,histidine, 4-hydroxy-L-proline, hydroxy-L-ricin, phenylalanine, proline,homoserine, methionine, 1-methyl-L-histidine, 3-methyl-L-histidine,L-lanthionine, L-ricin, L-leucine and the like. Of these, use of glycineis particularly preferable. The aforementioned amino acid can also beused in combination of 2 or more kinds.

Furthermore, peptide which can be used for the present invention isoligopeptide bonded by peptide bonds of 2 to 10 amino acids andpolypeptide bonded by peptide bonds of more than 2 to 10 amino acids.

Concrete examples of such peptide include carnosine, dultathionine,diketopiperazine and the like.

Peptide can also be used in combination of 2 or more kinds.

As (C) the organic ampholyte according to the present invention, anorganic ampholyte which does not contain a metal is preferably used. Asa method for obtaining the above-mentioned (C) organic ampholyte, amethod for obtaining, for example, glycine can be cited as described inJP1998-130214A. Such a method comprising adding ammonia and carbondioxide to glycolonitrile in a solution and heating the mixture toobtain glycine which does not contain various metals including sodiumcan be cited. Furthermore, a method comprising removing a metal from theorganic ampholyte by carrying out distillation purification or ionexchange can be properly cited.

(Amide Compound)

The coating liquid of the present invention comprising a configurationincluding an amide compound as described above is also a proper example.There is an advantage such that a porous silica film having muchexcellent smoothness is obtained from the coating liquid comprising anamide compound. Furthermore, the amide compound according to the presentinvention does not include (C) the organic ampholyte. As such an amidecompound, an amide compound as described in JP2003-89513A can bepreferably cited.

Of the aforementioned amide compounds, an amide compound having itsboiling point of less than 200° C., preferably not less than 150° C. andless than 200° C. is preferably used. When the boiling point is lessthan 200° C., it is easy to remove the amide compound during thepreparation of the porous silica film to be described later so that aporous silica film having uniform micropores regularly disposed can beeasily obtained. As such an amide compound, N,N-dimethylacetamide isparticularly preferable.

In the present invention, as the amide compound, use of an amidecompound which does not contain a metal is preferable. As a method forobtaining such an amide compound, a method comprising removing a metalfrom the amide compound by carrying out distillation purification, ionexchange or the like. Furthermore, as such an amide compound, an amidecompound for use in an electronic material that has been generallycommercialized can be used.

[Method for Preparing a Coating Liquid for Forming Porous Silica]

The coating liquid for forming porous silica according to the presentinvention can be prepared in accordance with all known preparationmethods of coating liquids. As a preferred example, a method forpreparing a coating liquid which comprises a partialhydrolysis-condensation product of the alkoxysilane compound, (B) thesurfactant and (C) the organic ampholyte, and satisfies the metalcontent of not more than 50 ppb is described.

An operation may be preferably carried out in a clean room that has beenmanaged not to allow a metal to enter into a solution. A vessel or anapparatus is used, which is subjected to a demetallization treatment bywashing with a 10 weight % sulfuric acid, and then washing withdemetallized water. Furthermore, a raw material does not contain ametal, i.e., an impurity or the aforementioned raw materials such as anacid catalyst, water, an organic solvent, an amide compound and the likeare used in this example, in addition to (A) the alkoxysilane compound,(B) the surfactant, (C) the organic ampholyte which are subjected to ademetallization treatment for removing a metal, i.e., an impurity.

Firstly, in the presence of (B) the surfactant, a reaction solution isobtained by carrying out a partial hydrolysis-dehydrative condensationreaction of (A) the alkoxysilane compound: Furthermore, the meaning of aliquid in the present invention includes a solution, a suspension, anemulsion and the like in some cases. The hydrolysis-dehydrativecondensation reaction is carried out in the presence of an acid catalystand water. It is preferably carried out further in the presence of anorganic solvent.

More specifically, the following methods can be cited,

(1) a method comprising feeding (A) an alkoxysilane compound, (B) asurfactant, an acid catalyst, water and an organic solvent as neededinto a reactor and stirring for several minutes to 5 hours or so;

(2) a method comprising feeding (A) an alkoxysilane compound, an acidcatalyst, water and an organic solvent as needed into a-reactor andstirring for about 10 minutes to 5 hours, subjecting (A) thealkoxysilane compound to a partial hydrolysis-dehydrative condensationreaction, adding (B) a surfactant thereto, and stirring for severalminutes to 5 hours or so; and the like.

The aforementioned components may be used in combination of 2 or morekinds in advance. For example, use of hydrochloric acid can beconsidered as using a mixture of an acid catalyst and water. In theabove method, each component can be divided into several parts and usedon a plurality of occasions. In particular, water is preferably dividedinto several parts and used on a plurality of occasions since an effectof the self-life stability is highly enhanced.

It is desired that water is used in the amount of from 0.5 to 20 moles,preferably in the amount of from 1 to 20 moles, and more preferably inthe amount of from 1.2 to 15 moles, based on 1 mole of (A) thealkoxysilane compound (in terms of a silicon atom).

When water is divided into several parts and used on a plurality ofoccasions, the number of the use of water is not particularlyrestricted, but it is desirable to add water used for the first time nthe amount of from 0.10 to 0.30 mole, preferably in the amount of from0.12 to 0.30 mole, and further preferably in the amount of from 0.15 to0.30 mole, based on 1 mole of an alkoxy group of (A) the alkoxysilanecompound.

When the amount of the above water is more than 0.30 mole, based on 1mole of an alkoxy group of (A) the alkoxysilane compound, gelation ofthe alkoxysilane compound takes place in some cases. Furthermore, thetime when water is used for the first time is preferably before (B) thesurfactant is used. On the other hand, the time for the use of (B) thesurfactant is preferably before water is used for the second time.

The time for the addition of water is desired to be after water used forthe first time is consumed in the amount of not less than 0.10 mole andpreferably not less than 0.12 mole by the hydrolysis, based on 1 mole ofan alkoxy group of (A) the alkoxysilane compound. The amount of waterconsumed by the hydrolysis can be confirmed generally by the KarlFischer analysis. When water used for the first time is consumed in theamount of not less than 0.10 mole, based on 1 mole of an alkoxy group of(A) the alkoxysilane compound, by the hydrolysis of (A) the alkoxysilanecompound, the number of addition of water thereafter and the time ofaddition are not particularly restricted.

In this manner, after the hydrolysis-dehydrative condensation reactionreaction of (A) the alkoxysilane compound is progressed, the resultantcomes in contact with (B) the surfactant. Then, the effect ofadditionally used water is small in many cases. This is consideredbecause the liquid is relatively stabilized by the interaction betweenthe alkoxysilane compound which is properly subjected to thehydrolysis-dehydrative condensation reaction and (B) the surfactant.That is, a method comprising using water on a plurality of occasionslike this is advantageous since a coating liquid which is excellent inthe self-life stability can be obtained, as compared to a methodcomprising using water at a time.

The aforementioned acid catalyst is used in the amount of from 0.001 to0.05 equivalent mole, based on 1 mole of the alkoxysilane compound (interms of a silicon atom). The organic solvent is not particularlyrestricted, but it is used in the amount to have a volume of from 3 to20 times, (A) the alkoxysilane compound.

Next, the thus-obtained reaction solution comes in contact with (C) theorganic ampholyte. Of course, (C) the organic ampholyte may be used inthe middle of the above reaction. In the present invention, (C) theorganic ampholyte plays an important role in enhancing the self-lifestability. Because the zeta potential of a partial condensation productof (A) the alkoxysilane compound which is unstabilized by removing ametal ion present in a solution at the state of cation is usuallyconsidered to become stabilized by (C) the organic ampholyte. On theother hand, (C) the organic ampholyte might possibly hinder progress ofthe cooperative organization by the interaction between (B) thesurfactant and a partial condensation product of (A) the alkoxysilanecompound. For that reason, the self-life stability of the coating liquidtends to be greatly affected by the time of the above reaction solutioncoming in contact with (C) the organic ampholyte and the use of (C) theorganic ampholyte.

In the above method, there is a method comprising determining the timeof the use of (C) the organic ampholyte after the confirmation ofcondensation degree of (A) the alkoxysilane compound, but the elapsedtime after (A) the alkoxysilane compound comes in contact with an acidcatalyst, and water can be used instead. Since thehydrolysis-dehydrative condensation reaction of (A) the alkoxysilanecompound starts after (A) the alkoxysilane compound comes in contactwith the acid catalyst and water, the condensation degree can becontrolled by the reaction time.

The time of the use of (C) the organic ampholyte cannot be uniformlydefined because it is different depending on the types of alkoxysilanein use or the like, but (C) the organic ampholyte is required to be usedafter (A) the alkoxysilane compound, the acid catalyst, and water comein contact with one another, followed by stirring for 30 minutes to 24hours, preferably for 45 minutes to 12 hours and further preferably for1 to 4 hours.

When water is divided into several parts and used on a plurality ofoccasions, it is required that (C) the organic ampholyte is used afterfeeding of the last water is completed, followed by stirring for 0 to 24hours, preferably 0 to 12 hours, and more preferably for 0 to 4 hours.

In this manner, particularly by controlling the amount, the time of use,and the like of water, (B) the surfactant, and (C) the organicampholyte, the condensation degree of (A) the alkoxysilane compound canbe maintained in the optimum state so that the self-life stability ofthe obtained coating liquid can be further improved.

Meanwhile, the aforementioned (C) organic ampholyte is contained in theamount of from 0.1 to 6000 ppm, preferably in the amount of from 50 to5000 ppm, more preferably in the amount of from 100 to 4000 ppm in thecoating liquid. (C) The organic ampholyte comes in contact with thereaction solution, preferably followed by stirring for about 1 to 30minutes. When the content of (C) the organic ampholyte is within thisrange, the self-life stability of the obtained coating liquid isparticularly enhanced. So, even when the coating liquid is kept for along period of time, the obtained silica film exhibits high regularityin micropore alignment.

An amide compound as described above may further be fed into the coatingliquid of the present invention.

The time of the use of the amide compound is preferably the same as thatof (C) the organic ampholyte. Furthermore, the amide compound may be fedat the same time with (C) the organic ampholyte or may be fedseparately. Further, the time of stirring after the use of the amidecompound is not particularly restricted as far as the amide compound ispractically uniformly mixed in the coating liquid.

The amount of the aforementioned amide compound used is desired to becontained in the amount of from 1 to 60 volume %, and preferably in theamount of from 5 to 35 volume %, based on 100 volume % of the reactionsolution. When the amount of the amide compound added is within thisrange, the self-life stability of the coating liquid is particularlyenhanced and a porous silica film having excellent surface smoothnesscan be obtained at the same time.

In the present invention, after a coating liquid is prepared by using araw material containing a metal, a metal ion is removed from the coatingliquid to have a metal content of not more than 50 ppb in some cases.

The metal content of the thus-obtained coating liquid for forming poroussilica can be confirmed, for example, by carrying out the inductivelycoupled plasma atomic emission spectroscopy.

The composition, the degree of condensation or the like of the coatingliquid for forming porous silica of the present invention can be easilyanalyzed by the known analytic methods. Examples thereof includeelementary analysis, infrared spectroscopy, ultraviolet spectroscopy,nuclear magnetic resonance (NMR) spectrum, liquid chromatography, gaschromatography, gel permeation chromatography and the like.

[Method for Preparing Porous Silica and a Porous Film]

As a method for forming porous silica or a porous silica film using thecoating liquid for forming porous silica of the present invention,preparation methods using known coating liquids can be used withoutrestrictions. A method comprising coating a base material with theaforementioned coating liquid, followed by drying, and further removing(B) the surfactant, (C) the organic ampholyte, and the amide compound tobe used as needed by calcination or extraction can be cited.

Porous silica or the porous silica film obtained by using the coatingliquid of the present invention has low permittivity and high regularityin micropore alignment, and also has superior strength. A film thicknessof the porous silica film is not particularly restricted because itspreferred range is different according to the applications. For example,when the porous silica film is used as an interlayer insulating film,its film thickness is required to be from 0.1 to 1 μm, and preferablyfrom 0.2 to 1 μm. By regulating the film thickness to be within thisrange, a film with a proper thickness can be formed, in which no crackand no leak current occurs.

Furthermore, since the aforementioned porous silica film is prepared bythe coating liquid, even when it is exposed to an electric field, shiftin capacitance or voltage never occurs.

Furthermore, by measuring the aforementioned porous silica film by XRD(X-ray diffraction), the regularity in micropores can be evaluated.According to the present invention, as far as the obtained peak in therange of 2θ=0.7˜8° according to the XRD measurement is sharper, a poroussilica film having regularly disposed micropores and of uniform sizescan be obtained.

The XRD measurement is carried out by a focusing method using 40 kV, 20mA, Monochromator (Graphite (00002) surface) with the CuKα line. Theabove peak strength of the porous silica film of the present inventionvaries depending on its film thickness or the like so it is notparticularly restricted. But the ratio of the peak to the noise (S/Nratio) is desired to be not less than 3. The porous silica film of thepresent invention having the S/N ratio within this range can beconfirmed to have uniform micropores, so that it can be used for anoptically functional material, an electronically functional material orthe like.

The porous silica film according to the present invention has propertiesas described above, thus it can be used for an optically functionalmaterial or an electronically functional material such as an interlayerinsulating film, a molecular recording medium, a transparent conductivefilm, solid electrolyte, an optical waveguide, an LCD color member andthe like. In particular, it can be preferably used as an interlayerinsulating film. Further, it can be preferably applied to asemiconductor device (circuit) and the like.

EXAMPLES

The present invention is now more specifically illustrated below withreference to Examples. However, the present invention is not restrictedto these Examples.

Incidentally, the following raw materials were used in Example andComparative Examples.

<Tetraethoxysilane>

EL Grade: Si(OC₂H₅)₄, a product of Kojundo Chemical Laboratory Co., Ltd.

<Ethanol>

For electronic industry, a product of Wako Pure Chemical Industries,Ltd.

<Hydrochloric Acid>

For trace analysis, a product of Wako Pure Chemical Industries, Ltd.

<Poly(Alkylene Oxide) Block Copolymer>

70 g of HO(CH₂CH₂0)₂₀(CH₂CH(CH₃)O)₇₀(CH₂CH₂O)₂₀H (Pluronic P123,manufactured by BASF) was weighed and dissolved in 700 g of the ethanolfor electronic industry. The resulting solution was subjected to ionexchange using an ionic exchange resin (SK1BH) manufactured by NipponRensui Co., and removing ethanol by distillation. Poly(alkylene oxide)block copolymer subjected to a demetallization treatment in this mannerwas used.

<Water>

Demetallized water by a water purification system manufactured byMillipore Corporation was used.

<N,N-dimethylacetamide>

For electronic industry, a product of Kanto Chemical Co., Inc

<Glycine>

H₂NCH₂COOH, a product of Mitsui Chemicals, Inc.

According to a method as described in JP1998-130214A, ammonia and carbondioxide were added to glycolonitrile in a solution and the mixture washeated for synthesizing.

<Cetyltrimethylammonium Chloride>

TCl-EP: C₁₆H₃₃(CH₃)₃N.Cl, a product of Tokyo Kasei Kogyo Co., Ltd.

Furthermore, the metal content and the preservation stability of acoating liquid for forming a porous silica film were measured asfollows.

<Measurement of the Metal Content of a Coating Liquid for Forming aPorous Silica Film>

A coating liquid with indium added as an internal standard substancethereto was analyzed using an inductively coupled plasma mass analyzer(7500S, a product of Agilent Technologies Inc.) into which a mixed gasof argon and oxygen was fed, and the metal content was quantitativelyanalyzed by the calibration method.

<Measurement of the Preservation Stability of a Coating Liquid forForming a Porous Silica Film>

Using a coating liquid right after the preparation thereof, a poroussilica film was prepared. The peak strength of this film by the XRDmeasurement was measured and defined as S0 as a reference value. Then,the coating liquid was put into a closed vessel, and stored at 10° C.After it was stored for a certain period of time, a film was preparedfrom the coating liquid taken out of the vessel. The peak strength ofthis film by XRD was measured, and defined as S1. The values of S0 andS1 were average values measured at three arbitrary points of theobtained films.

The ratio of the peak value of the measured value S1 to that of thereference value S0 (S1/S0) was defined as s (stability index). As far asthe value of s is larger, such a value indicates that the regularity ishigh and the stability of the coating liquid for forming a silica filmis high.

Furthermore, the preservation stability of the porous silica film of thepresent invention was also confirmed by preparing a film from thecoating liquid taken out of the vessel after the coating liquid was keptat 10° C. and kept for a predetermined period of time, and measuring itsdielectric constant. To measure the dielectric constant, aluminumelectrodes were prepared on a surface of the porous film on thesubstrate and a back surface of a silicon wafer used as a substrate bythe deposition method. The dielectric constant was measured according toa usual method at 25° C. under an atmosphere of a 0% relative humidityat a frequency of 100 kHz. The stability of the coating liquid forforming a silica film can be evaluated by the changing rate representedby the following formula,Δd=|d _(n) −d ₁ |/d ₁

wherein d₁ is the permittivity of a silica film obtained from thecoating liquid stored for one day; and d_(n) is the permittivity of asilica film obtained from the coating liquid stored for n days. Thesmaller the Δd value is, the higher the preservation stability is.

Example 1

In a clean room managed not to allow a metal to enter into a solution, acoating liquid was prepared. A vessel or an apparatus in use wassubjected to a demetallization treatment by washing with a 10 weight %nitric acid, and by washing with demetallized water. Firstly, 10.0 g oftetraethoxysilane and 10 mL of ethanol were mixed at a room temperature,followed by stirring. Then, 1.0 mL of 1N hydrochloric acid(corresponding to 0.02 mole of hydrochloric acid and 1 mole of water,based on 1 mole of tetraethoxysilane; further, corresponding to 0.25mole of water, based on 1 mole of the ethoxy group) was added theretoand the mixture was further stirred at 50° C. for one and a half hours.Subsequently, a solution obtained by dissolving 2.8 g of poly(alkyleneoxide) block copolymer (corresponding to the amount of the W_(CR) valueto be 1.0) in 40 mL of ethanol was added to the aforementioned liquidand mixed. Then, 8.0 mL of water (9.2 mole, based on 1 mole oftetraethoxysilane) was added thereto, followed by stirring for 50minutes, and then 20 mL of N,N-dimethylacetamide was added and mixed,following by further stirring for 40 minutes. Glycine was added to theobtained coating liquid such that 800 ppm of glycine is contained andthe mixture was stirred. A transparent homogeneous coating liquid wasobtained. The metal content of the coating liquid was measured. As aresult, the metal content was not more than 11 ppb.

1.5 mL of this coating liquid was put on a surface of a silicon waferhaving a diameter of 2 inches, rotated at 2000 rpm for 60 seconds toprepare a coated film (wet condition) on the surface of the siliconwafer. The obtained coated film was dried at 100° C. for 60 minutes andfurther calcined in the air at 400° C. for 180 minutes to prepare afilm. The obtained film was analyzed by X-ray. As a result, the film hada periodic hexagonal structure with area intervals of 7.0 nm.

This coating liquid was put into a closed vessel, and left alone at 10°C. for 1 day, 7 days, 15 days and 31 days. Then, the coating liquid wastaken out of the closed vessel, and a film was prepared by coating thesurface of the silicon wafer under the same conditions as describedabove. The results of X-ray diffraction measurement of the obtained filmare shown in FIG. 1.

The film obtained from the aforementioned coating liquid was confirmedto have kept its structural regularity high, regardless of the shelflife of the coating liquid.

Further, using the coating liquid which was put into the closed vesseland left alone at 10° C. for 1 day, 15 days and 31 days, the dielectricconstant of a film prepared under the same conditions as described abovewas measured. The results are shown in FIG. 2.

The permittivity (d1) of the silica film obtained by using the coatingliquid stored for one day was 2.0 at the dry state. Furthermore, thefilm obtained form the above coating liquid had Δd suppressed to within10% regardless of the shelf life of the coating liquid, so the abovecoating liquid was confirmed to have very high stability.

Comparative Example 1

A coating liquid was prepared and evaluated under the same conditions asin Example 1, except that glycine in Example 1 was not added. The metalcontent of the coating liquid was 7.4 ppb.

A film obtained from the coating liquid right after the preparationthereof had a periodic hexagonal structure with area intervals of 7.0nm.

The relationship between the X-ray diffraction measurement results andthe shelf life of the coating liquid is shown in FIG. 1. By storing thecoating liquid just for one day, it was found that the structuralregularity of the obtained film was considerably deteriorated.

The relationship between the dielectric constants and the shelf life ofthe coating liquid is shown in FIG. 2. The permittivity d₁ was 2.9.Further, the change in the permittivity in relative to the shelf lifewas abrupt so that the coating liquid was confirmed to be very unstable.

Comparative Example 2

A coating liquid was prepared and evaluated in the same manner as inExample 1, except that cetyltrimethylammonium chloride was added insteadof glycine in Example 1. The content of various metals in the coatingliquid was not more than 20 ppb.

A film obtained from the coating liquid right after the preparationthereof had a periodic hexagonal structure with area intervals of 7.0nm.

The relationship between the X-ray diffraction measurement results andthe shelf life of the coating liquid is shown in FIG. 1. The filmobtained from the coating liquid had maintained the structuralregularity relatively high regardless of the shelf life, whereas when 15days of the shelf life passed, it was found that the regularity began tobe deteriorated.

The relationship between the dielectric constants and the shelf life ofthe coating liquid is shown in FIG. 2. The dielectric constant alsobegan to change when 15 days of the shelf life passed. At the 31st day,Δd became 20%. Thus, it was found that the self-life stability wasdeteriorated.

From the above results, it became clear that the coating liquid forforming porous silica of the present invention had superior stabilityand the obtained porous silica or the porous silica film had properproperties for use in an interlayer insulating film having highregularity in micropores and low permittivity.

INDUSTRIAL APPLICABILITY

According to the coating liquid for forming porous silica and thepreparation method of the aforementioned coating liquid according to thepresent invention, a coating liquid exhibiting superior self-lifestability can be provided. Furthermore, the porous silica film of thepresent invention prepared by using the aforementioned coating liquid,even when it is exposed to an electric field, never causes shift incapacitance or voltage, has regularly disposed uniform micropores andthe like. Such excellent properties of the porous silica film do notchange so that the porous silica film can be properly used as anoptically functional material or an electronically functional material.Accordingly, the coating liquid for forming porous silica of the presentinvention as a raw material of porous silica is greatly meaningful onindustrial scale.

1. A coating liquid for forming porous silica comprising: (A) analkoxysilane compound; (B) a surfactant; and (C) an organic ampholyte,and having a metal content of not more than 50 ppb.
 2. The coatingliquid for forming porous silica according to claim 1, wherein (C) theorganic ampholyte is (C1) an amino acid and/or peptide.
 3. The coatingliquid for forming porous silica according to claim 1, wherein (C) theorganic ampholyte is contained in the amount of from 0.1 to 6000 ppm inthe coating liquid for forming porous silica.
 4. The coating liquid forforming porous silica according to claim 1, wherein (A) the alkoxysilanecompound is a partial hydrolysis-condensation product of (A1) analkoxysilane compound.
 5. The coating liquid for forming porous silicaaccording to claim 1, wherein the W_(CR) value defined by the followingformula is in the range of 0.5 to 3.0,W _(CR) =W _(C)/(60.09×M _(Si))   (Formula 1) wherein W_(C) represents amass of (B) the surfactant (unit: gram); and M_(Si) represents the molaramount of (A) the alkoxysilane compound in terms of silicon.